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Britain's Prince William, right, and Kate, Duchess of Cambridge, hold the Prince of Cambridge on July 23, 2013, as they pose for photographers outside St. Mary's Hospital's Lindo Wing in London.

Credit: AP Photo/Kirsty Wigglesworth

Prince William and Kate, Duchess of Cambridge -- known as the Earl and Countess of Strathearn in Scotland -- visit the Emirates Arena on April 4, 2013, in Glasgow, Scotland.

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Prince William and Kate, Duchess of Cambridge, visit the Singapore Botanical Gardens on Sept. 11, 2012, in Singapore during a Diamond Jubilee tour of South East Asia and the South Pacific.

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Kate, Duchess of Cambridge, and Prince William embrace while watching a cycling event during the London 2012 Olympic Games at Velodrome on Aug. 2, 2012, in London.

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Prince William and Kate, Duchess of Cambridge, applaud Switzerland's Roger Federer from the Royal Box on Centre Court during the 2012 Wimbledon Championships tennis tournament at the All England Tennis Club in Wimbledon, London, on July 4, 2012.

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Prince William and Kate, Duchess of Cambridge, are seen on the balcony of Buckingham Palace after the service of thanksgiving at St.Paul's Cathedral on June 5, 2012, in London, during the celebrations for Queen Elizabeth II's Diamond Jubilee.

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Prince William and Kate, Duchess of Cambridge, walk the red carpet with an umbrella as they attend the U.K. premiere of "War Horse" in London on Jan. 8, 2012.

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Prince William and his wife Kate the Duke and Duchess of Cambridge arrive on Aug. 19, 2011, for a visit to Summerfield Community Centre in Winson Green, Birmingham, England. The couple visited the community center in the English city of Birmingham, where three men were killed in a hit-and-run during last week's violence, they spent 15 minutes speaking with families of the victims and greeted emergency workers and local residents at the center.

Credit: AP Photo/PA/Rui Vieira

Kate, Duchess of Cambridge congratulates her husband, Prince William, Duke of Cambridge, at the trophy ceremony after his team won the charity polo match at The Santa Barbara Polo & Racquet club on Saturday, July 9, 2011, in Carpinteria, Calif.

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Prince William hugs his wife Kate, Duchess of Cambridge, after competing against her team in a dragon boat race at Dalvay-by-the-Sea on Prince Edward Island as part of their royal tour of Canada, Monday, July 4, 2011.

Credit: AP Photo/Robert F. Bukaty

Prince Harry, Kate, Duchess of Cambridge and Prince William leave the church after the royal wedding of Zara Phillips and Mike Tindall on July 30, 2011, in Edinburgh, Scotland.

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Prince William and wife Kate, Duchess of Cambridge, watch Britain's Andy Murray play Frenchman Richard Gasquet on Day 7 of the 2011 Wimbledon Tennis Championships at the All England Tennis and Croquet Club, in London, June 27, 2011.

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Prince William and Kate, Duchess of Cambridge, walk through Victoria Barracks to attend a ceremony as part of Britain's third Armed Forces Day activities, on June 25, 2011, in Windsor, England.

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In this handout image, supplied by St. James's Palace, Prince William and wife Catherine, Duchess of Cambridge, pose for an official tour portrait for their trip to Canada and California in the garden of Clarence House, on June 3, 2011, in London. The 10-day trip marks William and Kate's first official overseas tour as a married couple.

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Prince William and Kate, Duchess of Cambridge, stand on the balcony of Buckingham Palace after the Trooping the Colour parade on June 11, 2011, in London.

Prince William and Kate, Duchess of Cambridge, arrive at a charity event for Absolute Return for Kids, ARK, in central London, June, 9, 2011. The charity gala was the first official engagement for the royal couple since they were married on April 29.

Prince William, left, enjoys a joke with girlfriend Kate Middleton, right, on his birthday at Beaufort Polo Club on June 21, 2008, in Tetbury, England.

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Britain's Prince William and girlfriend Kate Middleton are photographed at RAF Cranwell, England, on April 11, 2008, after William received his RAF wings from his father, the Prince of Wales.

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Britain's Prince William, center foreground, and Prince Harry, second left foreground, and guests Chelsy Davy, left foreground, and Kate Middleton, third row on the right, watch the Concert for Diana at Wembley Stadium on July 1, 2007, in London.

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Prince William, right, stands beside girlfriend Kate Middleton in the paddock enclosure on the first day of the annual Cheltenham Race Festival at Cheltenham Race course, England, March 13, 2007.

Prince William of Wales and his girlfriend Kate Middleton leave the night club Boujis on Jan. 5, 2007, in London.

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In this image made available by in London, March 7, 2011, by The Middleton Family, Prince William and Kate Middleton pose together following their graduation from St. Andrews University, Scotland, on June 23, 2005.